My invention relates generally to a marine riser system for use in the drilling of subsea wells from a floating platform on a drilling vessel. More particularly, it relates to improvements in such systems employing dual gradient drilling techniques.
In the conventional drilling of a subsea well, a wellhead at the ocean floor connects to the platform at the surface by a riser system which includes, from bottom to top, a blowout preventer stack on the subsea wellhead, a lower marine riser package, a riser string made up of the individual riser joints, a telescopic joint and a diverter at the vessel. The purpose of the riser system is to control the well at the ocean floor, to guide the drill string and other tools into and out of the well, and to return the drilling mud and cuttings back to the drilling vessel for processing.
The weight of the riser string is often supported by a combination of buoyancy modules carried by the individual riser joints and tensioning equipment attaching the upper end of the riser string to the drilling vessel. The need for support includes the weight of the drilling mud column which is circulated through the drill string, out the bit, and up the annulus between the drill string and the riser back to the drilling vessel.
The drilling mud cools, lubricates and cleans the drill bit and carries the drilled cuttings to the surface, and the mud""s hydrostatic weight controls the well bore pressure. The mud is an engineered fluid, so that if the hydrostatic pressure is too high, the drilling mud may be lost and damage the formation. If the hydrostatic pressure is too low, formation fluids and gasses may flow into the well, where they must be controlled by the blowout preventer equipment.
In deep subsea wells, it is difficult to control the hydrostatic head of the drilling mud, and small changes in the density of the drilling fluid can cause large differences in down hole pressure. In one procedure proposed to address this problem, known as riserless drilling, the drilling mud is returned to the drilling vessel by being either pumped and/or gas lifted from the subsea BOP to the surface. This is an attempt to remove the effects of the hydrostatic head from the top of the subsea wellhead, and usually accomplished by having a second mud return riser in addition to the annulus within the drilling riser through which the drill string extends. Since most drilling vessels can only handle one riser at a time, this is a time consuming and difficult task.
Another procedure is known as underbalanced or dual gradient drilling in which nitrogen gas is added to the drilling fluid at the subsea level. The density of the aired up mud can then be controlled in an attempt to give the desired hydrostatic head at the subsea wellhead. In this system, the mud also returns to the drilling vessel through the annulus between the drill string and the riser, and a rotating blowout preventer is required at the surface to contain the gas in the annulus. In true underbalanced or dual gradient drilling, the pressure is normally less than the formation pressure. Under these conditions, the rate of drill penetration into the formation is greatly increased.
A co-pending patent application, Ser. No. 09/618,883, filed Jul. 18, 2000, entitled xe2x80x9cSystem and Method for Drilling Deep Water Subsea Wellsxe2x80x9d, and assigned the assignee of this application, discloses a procedure for handling the returning drilling mud without the shortcomings of the above described procedures. More particularly, it not only avoids the need for a second riser, as well as the obvious risks of collapse of the riser pipe attendant to underbalanced drilling, but also reduces the flow area available to the returning mud to thereby increase the velocity of the mud and thus aid in removing the cuttings from the well bore. The smaller column of mud reduces the tensioning requirements of the rig as well as the quantity of drilling mud to be carried on the rig. The system is also environmentally more friendly than conventional drilling, and, in the case of an unintentional drive off of the drill ship, reduces the amount of mud that would be lost to the ocean floor.
More particularly, the system is shown and described in the co-pending application as enabling the returning drilling fluid to be selectively diverted into a mud return line entering upwardly to the surface along the side of the riser string. Thus, in accordance with my invention, this diversion is accomplished by a system which includes an xe2x80x9cisolationxe2x80x9d tool installed between the LMRP and the lower end of the riser string so that, during dual gradient drilling, the heavy drilling mud may be isolated from the riser annulus, but which is of such construction as to enable the well bore to be easily, alternatively, and quickly entered.
Thus, in accordance with its illustrated and preferred embodiments, the isolation tool comprises a housing adapted to be connected as a lower continuation of the riser and having a bore through which the drill string may extend during the drilling of the well, an annular recess about the bore, and a side port below the recess for connecting the bore to a mud return line extending alongside of the riser and leading to the surface, as in the aforementioned application. An insert packer including a sleeve of elastomeric material is adapted to be lowered into and raised from a landed position in the bore opposite an actuator within the housing recess having a sleeve of elastomeric material which, when retracted, occupies a position in which the insert packer may be removed, forming a continuation of the bore so as to receive a drill string therethrough. When the insert packer is in place, the actuator sleeve is responsive to the supply of control fluid thereto from an outside source to engage and contract the sleeve of the insert packer about the drill string, so that the drilling fluid flowing upwardly in the annulus between the riser and drill string is directed into the side port in the housing bore. In response to the exhaust of the control fluid, the insert packer sleeve is free to expand to fully open the bore and the insert to be removed.
In one form, the actuator sleeve has metal rings at both ends to seal with the housing recess in order to contain the control fluid. The insert packer sleeve also includes upper and lower metal rings, and a pin is mounted on the housing for extension inwardly of the bore to support the lower ring of the insert packer opposite the actuator sleeve. More particularly, one of the rings has an outer recess and another pin is mounted in the housing for extension inwardly of the bore to engage in the recess to hold the packer insert down in the landed position.
In another embodiment of the invention adapted for use when the drilling string is rotating, the insert packer includes a carrier body having a recess about its inner side, and an elastomeric sleeve having a bearing ring at each end slidably rotatable within the body recess for rotation with the drill string when closed thereabout. More particularly, the actuator includes three vertically spaced elastomeric sleeves within the recess of the housing body and separated by spacer rings in position to be contracted by control fluid supplied from an outside source to close about the outside of the carrier body, and thus form upper and lower fluid pressure chambers.
Each of the spacer rings has a port therethrough leading to its adjacent pressure chamber, and the carrier body and bearing rings have passageways connecting at opposite ends to the upper and lower pressure chambers on the outer side of the sleeve within the recess of the carrier body, whereby control fluid may be supplied from an outside source into one port and out the other. Thus, when the actuator sleeves are contracted about the carrier body, control fluid will circulate through the passageways in order to contract the actuator sleeve about the insert packer sleeve and thus contract it about the drill pipe. Due to its rotating support within the carrier body, the sleeve of the insert packer is free to rotate with the pipe. The relief of such control fluid permits the sleeve of the actuator, and thus the sleeve of the insert packer, to expand to permit the insert packer to be retrieved.
Preferably, the carrier body has a main portion in which one end of the recess is formed, and a retainer portion removably connected to the main portion and on which the other end of the recess is formed, whereby the sleeve and bearing rings of the insert packer may be installed or removed upon removal of the retainer portion. More particularly, a pin is mounted on the housing for movement inwardly of the bore to support the carrier body, and there is a recess in the outer side of the body into which another pin mounted on the housing body is adapted to be moved to hold the packer insert in landed position and out of the recess to free the carrier body and insert packer sleeve for removal from the housing bore.